The present invention relates to a scattered radiation correcting technology to be implemented in an X-ray computed tomography (CT) system that produces an X-ray tomographic image of a subject through X-irradiation.
The major object of X-ray CT systems is to acquire projection data provided by X-rays having passed through a subject, and to reconstruct an X-ray tomographic image from the projection data. More particularly, first, a subject is asked to lie down on a patient couch, and transported to a bore of a gantry. A rotary assembly of the gantry in which an X-ray detecting mechanism including an X-ray tube and X-ray detectors is incorporated as an integral part thereof is driven to rotate. X-rays are irradiated to the subject from different angles, and X-rays irradiated at the respective angles and transmitted by the subject are detected. An operator console receives the detected data (projection data) and reconstructs an X-ray tomographic image by performing arithmetic operations. The sequence of steps for detecting X-rays is generally referred to as a scan.
Moreover, a so-called multi-slice X-ray CT system is known as a system having a plurality of arrays of detectors arranged in the direction of transportation in which the patient couch is transported. The multi-slice X-ray CT system has the merit that projection data representing a plurality of sections can be acquired by performing one scan. On the other hand, when the number of arrays of detectors increases, the adverse effect of scattered radiation oriented in the direction of the arrays of detectors cannot be ignored. For example, when the number of detector arrays is one or two, the majority of scattered radiation is radiated in directions in which no detector is present. It is therefore unnecessary to take account of the adverse effect of scattered radiation oriented in the direction of the detector arrays. However, when the number of detector arrays increases (for example, 32 or more), the scattered radiation oriented in the direction of the detector arrays falls on the detectors. Moreover, the amount of scattered radiation falling on each detector array is not even but markedly different especially between the edge-side detector array and the central detector array.
Incidentally, measures have been taken to cope with scattered radiation oriented in the direction of channels assigned to detectors in the past. For example, a collimator or a grid is disposed on the border of adjoining channels in order to prevent incidence of scattered radiation. However, this method brings about a decrease in an effective area of an X-ray detection surface. Moreover, an issue of an angle at which an X-ray tube is looked up cannot be ignored, and the use efficiency of X-rays is degraded. Moreover, an increase in the cost of hardware cannot be avoided because the collimator must be disposed highly precisely.
Consequently, scattered radiation oriented in the direction of the detector arrays should not be coped with by modifying hardware but should be coped with using software of data correction. Technologies for compensating the adverse effect of scattered radiation oriented in the direction of the detector arrays using software of data correction include, for example, the one described in Patent Document 1 presented later. Patent Document 1 discloses an X-ray computed tomography system that acquires scattered radiation data from output data of arrays of X-ray elements which are included in a multi-slice X-ray detector having, for example, eight arrays of X-ray elements and to which X-rays are not directly irradiated.
[Patent Document 1]
Japanese Unexamined Patent Application Publication No. 2000-197628
However, correction of scattered radiation to be performed in the X-ray computed tomography system disclosed in Patent Document 1 is not devised in consideration of the situation that an amount of scattered radiation entering each detector array is not even. Therefore, there is the room for realization of highly precise correction of scattered radiation.